A recognized problem in the art of projecting photographic transparencies onto a viewing screen is that of a loss of focus caused by a transparency popping, buckling, or otherwise warping from heat deformation. In a typical projector, the distance between the transparency and the projection lens is varied so as to focus the projection of the transparency on the viewing screen. This focusing is normally done when the transparency is first introduced into the projector. However, as the slide absorbs the heat from a projection lamp, it warps or buckles, changing the distance to the projection lens and thus defocusing the projection on the screen.
Several methods of partially correcting the focus of the projection onto the viewing screen are currently known. In manually focussed projectors, the operator simply readjusts the distance between the center of the transparency and the projection lens (i.e. refocuses the projector) so that the center of the projection is in focus.
U.S. Pat. No. 3,628,857 to Harvey (assigned to the same assignee and issued to the same inventor as the present invention) shows an autofocus transparency projector wherein apparatus is included for automatically measuring the distance to the viewing screen, and for automatically measuring and adjusting the distance between the transparency and the projection lens. The distance between the transparency and the projection lens is constantly monitored and adjusted so as to maintain the center of the transparency in focus on the viewing screen.
U.S. Pat. No. 2,947,215 to Mitchell shows a projector which compensates for popping of a transparency by shifting a glass wedge so as to vary a focal plane of a projection lens. This varying of the focal plane functions to refocus the center of the transparency on the viewing screen.
All of the above-described apparatus for focusing popped or warped transparencies fall short of the optimum, desired perforamnce because they only correct the focus of the center of the popped transparency. A human operator manually refocussing a projector to correct for a popped transparency will inherently focus on the transparency center. The automatic projectors as shown in Harvey and Mitchell include apparatus for automatically refocussing the centers of the popped slides. Because of the high f-stops of the projection lenses used in these projectors, the depth of field of the projection is small and the edges of the popped transparencies remain out of focus. This small depth-of-field prevents the proper focus of all regions of a popped transparency. Even if an attempt was made to refocus on a region other than the center, some region of the transparency would remain out of focus.
U.S. Pat. No. 3,416,859 to Badalich shows a transparency projector wherein slide warpage is prevented by direct mechanial pressure between a glass lens and a transparency. Such an arrangement suffers, however, from a variety of disadvantages inherent in the operation of hot, touching surfaces. These disadvantages include marring of the slide, possible heat damage to the slide, and sticking between the slide and the glass lens.